[keysever/lib/native] introduce gate for tagging farcaster channel work

Summary:
This gate will be used to guarantee that nobody (internally + externally) creates a Farcaster channel tag and thus has a blob holder on their client on prod.

Depends on D11772

Test Plan: Confirmed that when GATE_TAG_FARCASTER_CHANNEL is true we can't access the UI + also confirmed that the create_or_update_farcaster_channel_tag would immediatly throw an error

Reviewers: tomek, atul, inka, ashoat

Reviewed By: ashoat

Subscribers: ashoat, tomek

Differential Revision: https://phab.comm.dev/D11808